Such cables are used in various fields in industry, such as, for example, the automobile industry, where they are assembled in bundles for electrically powering various pieces of equipment. Such cables thus need to be as lightweight as possible, and to be compact, while retaining good mechanical strength.
Conventionally, such cables are made up of a plurality of copper strands, generally twisted together to form a twisted strand so as to increase the flexibility of the cable, and surrounded by an insulating sheath, e.g. obtained by extrusion. FIG. 1 shows an example of such a cable 1, seen in cross-section, and made from seven identical copper strands 20 surrounded by an insulating sheath 30 of circular section. To give a concrete idea, the diameter of the cable is typically about 1.6 millimeters (mm) and each of the copper strands 20 presents a diameter of about 0.3 mm.
Other cables of structure similar to that of FIG. 1, but having some other number of copper strands, e.g. nineteen strands, are also known.
The advantages of a cable having the same structure lie essentially in the simplicity of its method of fabrication, and also in the fact that it is suitable for crimping reliably to connectors. It suffices to strip the cable locally by removing a portion of the insulating sheath 30 where it is desired to place a connector, and then to compress the bushing of the connector mechanically around the stripped section of cable. In addition, copper intrinsically presents good mechanical strength in traction.
In contrast, it has been found that the above cable makes use of a quantity of copper that is excessive compared with the real requirements corresponding to the amount of electricity that is to be conveyed by the cable. More precisely, about half of the copper in the above cable structure is used for providing the cable with traction strength, and also for guaranteeing effective crimping.
Unfortunately, copper is becoming ever more expensive, and it is important to find new cable structures that minimize the quantity of copper used to the smallest possible amount.
Various composite cable solutions are already known in which copper strands are combined with a core of non-conductive material. In particular, U.S. Pat. No. 7,145,082 describes a control cable in which a large quantity of conductor wires, e.g. made of copper, are twisted around a central core made up of a multistrand polymer of the aramid fiber type.
That type of cable makes it possible to reduce significantly the quantity of copper that is used, down to the amount that is just sufficient for good signal transmission, while conserving very good mechanical strength in traction because of the use of the aramid. In contrast, the number of copper strands remains very large compared with the solution shown in FIG. 1 where the copper strands are disposed in a single concentric layer around the central strand.
Merely replacing the central copper strand in the structure of FIG. 1 with a multifilament polymer core of the kind described in U.S. Pat. No. 7,145,082 is not appropriate since such a cable does not provide sufficient guarantees concerning crimping operations. Once such a cable is stripped for a crimping operation, the copper strands splay apart from one another a little, and some of the polymer filaments making up the core run the risk of escaping radially between two copper strands. This situation is shown diagrammatically in FIG. 2 which is a cross-section of such a cable after a portion of the insulating sheath 20 has been stripped. As can be seen, certain filaments of the core 40 made of multifilament polymer are to be found on the outside of the outer ring of copper strands 20. Thus, when the bushing of the connector is compressed around the stripped section of cable, these filaments become interposed between the copper strands and the bushing, thereby reducing the contact area relative to that required for proper transmission of the electrical signal.
Patent document EP 1 089 299 discloses a cable structure in which a plurality of strands of conductive material are twisted concentrically around a core made up of a plurality of reinforcing fibers embedded in a metal material. Such a cable is expensive to fabricate, in particular because it uses a matrix of metal material for embedding the fibers.
U.S. Pat. No. 5,159,157 also discloses a control cable in accordance with the preamble of claim 1 of the present application, in which the carbon fibers of the core are secured to a non-metallic unitary structure. More precisely, a vaseline type filler matrix fills all of the cavities between the carbon fibers and the strands of conductive material. Such a structure remains expensive to fabricate, because it uses said filler matrix.